Pattern recognition apparatus is widely used in modern manufacturing processes to position work items with precision. In the manufacture of hybrid integrated circuits, for example, such equipment is frequently incorporated in automatic wire bonding machines. Components must be positioned with acceptable precision before bonding takes place lest connections be mislocated and products result which either require repair or must be discarded. If too many products have to be repaired or discarded because of faulty wire bonding, a great many of the advantages of an automated manufacturing operation are lost.
U.S. Pat. No. 4,385,322 (R. A. Hubach et al.), issued May 24, 1983, discloses pattern recognition apparatus including a television camera for creating an image of a work item and a processor for converting the image of the work item to digital form. The processor uses a selected threshold to determine which grey level is to be digitized as black instead of white. The picture elements (referred to in the art as pixels) of the digitized image are compared with those of a digitized reference image of the work item stored in memory. The pattern recognition apparatus then not only generates offset coordinates giving the apparent location of the work item image with reference to a base location, but also generates a quality number representing the extent of the match between the pixels of the digitized work item image and those of the digitized reference image. If the quality number is at least as large as a predetermined quality reference level, the offset coordinates are accepted as correct and used to control a servo mechanism which positions the work item. A frequently encountered problem in practice is that, because of such factors as changes in ambient lighting, differences in the reflectivity of individual work items, or even degradation in camera sensitivity, the selected grey level threshold does not always yield the desired results. It is desirable, therefore, that something be done to avoid an unnecessarily high percentage of rejects.
In the past, this problem has been attacked in the operation of pattern recognition apparatus by going a step beyond simply generating a quality number once and comparing it with the predetermined reference level. If this first quality number was less than the reference level, the attempt failed and the offset coordinates generated with it could not be used. The equipment was then made to begin cycling step by step through a number of different grey level thresholds, generating new offset coordinates and new quality numbers until either a quality number was found which at least reached the reference level or the cycle was completed without success. If a new quality number was found which equaled or exceeded the reference level, the offset coordinates associated with it were accepted and used to position the work item. If no new quality number equaled or surpassed the reference level, the pattern was considered not found (a "no find") and the work item discarded. The no find decisions could be reduced by lowering the quality reference level, of course, but only at the cost of en increased likelihood of error.
It is desirable to improve the efficiency of such pattern recognition and increase the useful product yield as much as possible with little or no throughput time loss and with little or no increase in the likelihood of error.